Fractionation control



United States Patent 3,165,44 FRACTIGNATEON CGNTRQL Donald R. Wienecke, Berger, Tex, assignor to Phillips Petroleum Company, a corporation of Delaware Filed Mar. 1, 1962, Ser. No. 176,724 3 Claims. (Ql. 202-160) This invention relates to the computation of internal reflux in fractionation columns. In another aspect it relates to the control of fractionation columns in response to compuations of internal reflux.

It is known that even small changes in the temperature of reflux returned to a fractionation column can create disturbances which are so severe that control instruments employed on the column must be detuned to preserve stability. Unless the reflux temperature is controlled rather precisely, the overall control system can operate in an oscillatory manner. This situation has been observed on many fractionation columns equipped with conventional control instruments.

It has recently been discovered that the control of fractionation columns can be improved by computing and regulating the internal reflux in the column. Internal reflux constitutes the external reflux returned to the column plus the vapor which is condensed near the top of the column by sub-cooledexternal reflux. This internal reflux R can be computed from the following equation:

where R is the external reflux returned to the column, AT is the difference between the temperatures of the overhead vapor and the external reflux, and K is equal to the specific heat of the liquid on the top tray divided by m'lce Other objects, advantages and features of the invention should become apparent from the following detailed description, taken in conjunction with the accompanying drawing which is a schematic representation of the fractionation control system of this invention.

Referring now to the drawing in detail, there is shown I a conventional fractionation column 10 which is provided with a number of liquid-vapor contacting trays, three of which are shown schematically in the top section of the column. A feed mixture to be separated is introduced into column 10 through a conduit 11 at a predetermined rate which is maintained by a flow controller 12 that adjusts a valve 13. Steam, or other heating medium, is supplied to the lower region of column 10 through a conduit 14 at a predetermined rate which is maintained by a flow controller 15 that adjusts a valve 16. A kettle product stream is withdrawn from the bottom of column 10 through a conduit 18 which has a control valve 19 therein. The rate of kettle product withdrawal is regulated by a liquid level controller 20 which adjusts valve 19 so as to maintain a predetermined liquid level in the bottom of column 10.

Vapors are withdrawn from the top of column 10 through a conduit 22 which communicates with the inlet of a condenser 23. A suitable coolant is circulated through condenser 23 from a conduit 24 which has a control valve 25 therein. The outlet of condenser 23 is connected by a conduit 26 to a reflux accumulator 27. The vapors which are not condensed are withdrawn from accumulator 27 through a conduit 28 as an overhead product stream. A pressure controller 29 adjusts a valve 30 l in conduit 23 so that the vapors are withdrawn at such a the heat of vaporization of liquid on the top tray. Ap-

paratus which can be employed to solve this equation and to control fractionation columns in response thereto is described in detailin ISA Journal, June 1959, pages 34 et seq.

In most fractionation systems, the internal reflux computer described in the above-mentioned article provides efiicient control of the external reflux flow. However, this system does not always provide the desired accuracy if only a portion of the overhead vapors are condensed. When this occurs, the external reflux returned to the column does not have the same composition as the overhead vapors removed from the column. In accordance with the present invention, a novel system is provided for computing the internal reflux to be supplied to a column when only a portion of the overhead vapors are condensed. This computation is made from measurements of the rate of flow of the overhead vapors, the rate of flow of external reflux, the temperature of the overhead vapors, the temperature of the external reflux returned to the column, and the temperature of the vapors passed upwardly to the top tray in the column. Signals representative of these measurements are combined so as to provide a computed value of the internal reflux within the column. A signal representative of this internal reflux can then be employed to control the operation of the column to maintain desired steady-state conditions. This control can be made automatically or by an operator from the information provided.

Accordingly, itis an object of this invention to provide a system for computing the internal reflux in a fractionation column.

Another object is to provide control systems for fractionation columns, which systems utilize computations of the internal reflux in the columns.

Another object is to provide a method of and apparatus for computing internal reflux in fractionation systems wherein only a portion of the overhead vapors are condensed to supply external reflux.

rate as to maintain a predetermined pressure within accumulator 27. The condensate in accumulator 27 is returned to the top of column 10 through a conduit 31 to provide reflux to the column. A liquid level controller 32 an accumulator Z7 regulates valve 25 soas to maintain a predetermined liquid level in the accumulator. This assures that sufficient vapors are condensed to provide the required reflux to the column.

In order to explain the operation of the internal reflux computer and control system of this invention, an equation will be derived which is representative of the internal reflux in the fractionation column.

The first basic relationship employed in this derivation Q= where Q is the total enthalpy of a stream (B.t.u./hour, for example), W is the flow rate of the stream (pounds/ hour, for example), and H is enthalpy of the stream (l3.t.u./ pound, for example); The second basic relationship is where T is the temperature of a stream F., for example), C is a constant expressing the relationship between temperature and enthalpy of the stream, and C is a constant to adjust the enthalpy base to a reference temperature.

The heat balance at the top of the fractionator can be expressed as Qir=Qer+Qv Qoh where Q is the enthalpy of the internal reflux flowing downwardly from the top tray of the fractionator, Q is the enthalpy of the external reflux returned to the fractionator, Q, is the enthalpy of the vapor rising to the top tray, and Q is the enthalpy of the vapors removed from the top of the fractionator. The corresponding material balance can be expressed as WV: olf er+ lr where the subscripts have the same meanings as noted with regard to the Q terms.

Equation3 can be modified bysubstituting Equation 1 Y to obtain Winner /Hen+ Wv o- WaHoh The value of W of Equation 4 can be substituted into Equation 5 to obtain ir ir' erHer) i 7 efi" W11) 7 Equation 6 can be rearranged to obtain er er oner) v];[ sh oh] a HFHJ Hit-Hv Hu -1v 7 The quantity W is the internal reflux.

The computer of this invention solves Equation 7. t A

' temperature sensing element 46 is disposed in conduit 22 temperature transducer 41. This transducer can be any commerciallyiavailable instrument which is provided with both a=scale factor andan adjustablezero point. These two factorscanbe adjusted so that the output signal from transducer 41 is representative of H assuming the com- .7 position of the internal reflux does not change materially. t The output signal from transducer 41 is applied to the first input of a summing element 42 and to the first input of a subtracting element 43. Summing element 42 is supplied with a reference signal which is'represent'ative of "the heat of vaporization of the liquid in' the upper region of the column. This is added to the input signalI-I such I that the output signal from element 42, which is transmitted to'the first input of a multiplying and dividing element 44, is representative of thequantity H a A second temperature'detecting element 4-5 is disposed within column ltlto measure-the temperature of the liquid descending from the second tray from the top of the column because the temperature is indicative of the aver-5 I age temperature ofthe vapors to the top tray. This 'de-' f tecting element is connectedtojthe input of a temperature transduceri ld which is calibrated to provide an output.

3 signal representative'oi the quantity H This signal is an input of the element-44.

i applied to thesecond inputs of subtracting element 43 so forth inthe following-table;

i IBasis: 10,000 moles 0! feed] I Kettle Over-t Over- Ex- 7 Component Feed Product head head ternal 1 Vapor Product Reflux Oarbon dioxide; s 0 50 23. V 17 Hydrogen sulfide; 336 59 576 277 299 lvlethaue a 444 0- 582 444i 138 (3 Hydrocarbons" 1,580 80 2,828 1,600 1,328 0 Hydrocarbons. 3,646- 2;'(320 3,29 1, 026 2, 268 Isobutane -a 480' 4 151 a 1-9. 122 Normal Butane 1, G74 1, 620 331 54 277 Isopentane 438 t 437 15 .1 14 Normal Pentane 564 56 i 10 0 10V 06 Hydr0carb0ns 522 522' 1 0' t 1 G Hydrocarbon 283 283 I '0 3 0 0 V V p 10,600 6,636 7,838 3,364 4,474

Flow (gallons) 111,100 80,300, 44-, 200 Flow (Mcft) 2, 9305 1,280 Temperature F) 170 240 13D 9 95 from element 53 constitutes the. third input to summing element'49. Element 43 adds the output signalfromelevment 54 to the output signal from element 53 and sub separation thereof. It the computed internal reflux should deviate from this .desired'value, a signal is applied to controller'ti from network 4'9 which adjusts the setting of valve 61 to return the computed value of the internal reflux to the desired-value. For example, if the com-' puted value should fall below the-set pointgvalve 61 is opened further to increase the As a specific example of theoperation of this invention, reference is made to the Ease of columnlt) as adeethanizer. Typical rates of flow and compositions areset *The requiredenthalpy values for these and other fluid mixtures can readily be determined either from c'alcula- 45' t positions at the points of temperature measurement can tions or actual measurements of fluid samples; The corn:

.- be determined by withdrawing fluid samples from these points and analyzing the with drawn samples.

From the foregoing description it should be evident that an improved procedure is provided for computing internal reflux in fractionation columns. This procedure is particularly valuable whereonly part of the overhead vapors the. inputof a square root element 48.- Element 43 is calibrated such'that the output signal W5 is representative of the rate of flow of overheadvapors. The output a summing element 49.

signal from element 44% is'supplied toone of the inputs of A second differential pressure transmitter 50 nected across an orifice in reflux conduit 31. The, output 7 signal from transmitter 50 is applied through a square root elementSll to the input'of a subtracting element 52 and to the input of asecond tmultiplying and dividing ele-.

ment 53. The second input signal toelement 52 is the output of square root element 48. The output of element 52 is applied to thefirst'input of athird multiplying and dividing element 54. Network 54- also receives input signals from transmitter 46 and element 43.- The output of element 54 element 49.

constitutes the second inputto summing i to the'second input of element 53. The output-signal.

is conare condensed'to provide the external reflux. j invention has been-described in conjunction "with a 55" While the presently preferred embodiment, itshould be evident'that it is not limited thereto. r

What is claimed is: I j V 1." In a fractionation system wherein a fluid" mixture to be separated is introduced into a frac'tionationcolumn; a kettle product stream'is removed from the lower region of said column; vapors are withdrawn from the'upper region of'said column; aportionot said vaporsare condensed; and resulting condensate is returned to the upper region of saidcolumn as external reflux; a control system comprising means to sense the temperature of the vapors withdrawn from said column and to establish a first signal H in response thereto, said'signal H l being representa-t tive of-the enthalpy of the, internal reflux-in said column at a region immediatelybelowthe.point'of introduction of fexternal reflux towthe column; means responsive .to, said first signal to establish a second signalI-L', representative a h of the enthalpy of the vaporswithdrawn from. said col umn; means to sense the temperature of'said external reflux and .to establish a third signal H in response there- -to, saidtsigna'l H being representative of-the enthalpy a twelfth signal 5 I of said external reflux; means to sense the temperature in said column a short distance below the point at which said external reflux is returned and to establish a fourth signal H in response thereto, said signal H being representative of the enthalpy of vapor in said column at the region the temperature is sensed; means to measure the rate of fiow of overhead vapors from said column and to establish a fifth signal W representative thereof; means to measure the rate of flow of external reflux and to establish a sixth signal W representative thereof; means responsive to said first, second, third, fourth, fifth and sixth signals to establish a seventh signal W which is equal to the quantity and means responsive to said seventh signal to adjust the rate of flow of said external reflux.

' 2. The control system of claim 1 wherein said means to establish said seventh signal comprises means to subtract said fourth signal from said first signal to establish an eighth signal (E -E means to multiply said second signal" by said fifth signal and to divide the product by said eighth signal to establish a ninth signal means to multiply said third signal by said sixth signal and to idvide the product by said eigth signal to establish a tenth signal BX CI' a- 0 means to subtract said sixth signal from said fifth signal to establish an eleventh signal (W W means to multiply said eleventh signal by said fourth signal and to divide the product by said eighth signal to establish oh' er) v lr v) and means to add said tenth signal to said twelfth signal and to subtract said ninth signal from the sum to establish said seventh signal.

3. In a fractionation system wherein a fluid mixture to be separated is introduced into a fractionation column; a kettle product stream is removed from the lower region of said column; vapors are withdrawn from the upper region of said column; a portion of said vapors are condensed; and resulting condensate is returned to the upper region of said column as external reflux; apparatus for computing the internal reflux in said column comprising means to sense the temperature of the vapors withdrawn from said column and to establish a first signal H in response thereto, said signal H being representative of the enthalpy of the internal reflux in said column at a region immediately below the point of introduction of external reflux to the column; means responsive to said first signal to establish a second signal H representative of the enthalpy of the vapors withdrawn from said column; means to sense the temperature of said external reflux and to establish a third signal H in response thereto, said signal H being representative "of the enthalpy of said external reflux; means to sense the temperature in said column a short distance below the point at which said external reflux is returned and to establish a fourth signal H in response thereto, said signal H being representative or" the enthalpy of vapor in said column at the region the temperature is sensed; means to measure the rate of flow of overhead vapors from said column and to establish a fifth signal W representative thereof; means to measure the rate of flow of external reflux and to establish a sixth signal W representative thereof; and means responsive to said first, second, third, fourth, fifth and sixth signals to establish a seventh signal W which is equal to the quantity er u oh et) v] oh ah ir v ir v ir v References Cited in the file of this patent UNITED STATES PATENTS Morgan Jan. 23, 1962 'Lupfer' Feb. 6, 1962 OTHER REFERENCES 

1. IN A FRACTIONATION SYSTEM WHEREIN A FLUID MIXTURE TO BE SEPARATED IS INTRODUCED INTO A FRACTIONATION COLUMN; A KETTLE PRODUCT STREAM IS REMOVED FROM THE LOWER REGION OF SAID COLUMN; VAPORS ARE WITHDRAWN FROM THE UPPER REGION OF SAID COLUMN; A PORTION OF SAID VAPORS ARE CONDENSED; AND RESULTING CONDENSATE IS RETURNED TO THE UPPER REGION OF SAID COLUMN AS EXTERNAL REFLUX; A CONTROL SYSTEM COMPRISING MEANS TO SENSE THE TEMPERATURE OF THE VAPORS WITHDRAWN FROM SAID COLUMN AND TO ESTABLISH A FIRST SIGNAL HIR IN RESPONSE THERETO, SAID SIGNAL HIR BEING REPRESENTATIVE OF THE ENTHALPY OF THE INTERNAL REFLUX IN SAID COLUMN AT A REGION IMMEDIATELY BELOW THE POINT OF INTRODUCTION OF EXTERNAL REFLUX TO THE COLUMN; MEANS RESPONSIVE TO SAID FIRST SIGNAL TO ESTABLISH A SECOND SIGNAL HOV REPRESENTATIVE OF THE ENTHALPY OF THE VAPORS WITHDRAWN FROM SAID COLUMN; MEANS TO SENSE THE TEMPERATURE OF SAID EXTERNAL REFLUX AND TO ESTABLISH A THIRD SIGNAL HER IN RESPONSE THERETO, SAID SIGNAL HER BEING REPRESENTATIVE OF THE ENTHALPY OF SAID EXTERNAL REFLUX; MEANS TO SENSE THE TEMPERATURE IN SAID COLUMN A SHORT DISTANCE BELOW THE POINT AT WHICH SAID EXTERNAL REFLUX IS RETURNED AND TO ESTABLISH A FOURTH SIGNAL HV IN RESPONSE THERETO, SAID SIGNAL HV BEING REPRESENTATIVE OF THE ENTHALPY OF VAPOR IN SAID COLUMN AT THE REGION THE TEMPERATURE IS SENSED; MEANS TO MEASRURE THE RATE OF FLOW OF OVERHEAD VAPORS FROM SAID COLUMN AND TO ESTABLISH A FIFTH SIGNAL WOH REPRESENTATIVE THEREOF; MEANS TO MEASURE THE RATE OF FLOW OF EXTERNAL REFLUX AND TO ESTABLISH A SIXTH SIGNAL WER REPRESENTATIVE THEREOF; MEANS RESPONSIVE TO SAID FIRST, SECOND, THIRD, FOURTH, FIFTH AND SIXTH SIGNALS TO ESTABLISH A SEVENTH SIGNAL WIR WHICH IS EQUAL TO THE QUANTITY 